The present invention relates to processes for preparing and controlling the fractionation of chalcogenide alloys. More specifically, the present invention relates to processes for controlling the fractionation of chalcogenide alloys during the vacuum deposition thereof. In one embodiment, the present invention is directed to a process for controlling and suppressing the fractionation of chalcogenide alloys, and especially selenium alloys, which comprises the crystallization including the partial crystallization of said alloy, grinding and forming pellets thereof, and subsequently vacuum evaporating thereby enabling, for example, desirable homogeneous products. Another embodiment of the present invention comprises the provision of chalcogenide alloy pellets, the crystallization or partial crystallization of said alloy pellets, grinding and forming second pellets thereof, and subsequently vacuum evaporating. The products resulting from the process of the present invention can be selected as photoconductors in electrophotographic imaging systems, including xerographic imaging and printing processes.
Chalcogens and chalcogenide alloys, and their use in electrophotographic processes is known. Generally, the aforementioned photoconductors are prepared by known vacuum deposition, flash evaporation, and chemical vapor deposition methods. These methods possess disadvantages in some instances. Thus, for example, with vacuum deposited chalcogenide alloys the products obtained lack controllable reproducibility in their homogeneity thereby adversely affecting the electrophotographic electrical characteristics thereof. As the components selected for vacuum deposition usually have different vapor pressures, such components tend to separate during the vacuum deposition process causing undesirable inhomogeneity, or fractionation thereof. Also, in the vacuum deposition processes the components or species with, for example, high selenium content tend to evaporate more rapidly during the initial stages of deposition primarily, it is believed, because of their higher partial vapor pressure, thus resulting in uncontrolled, undesirable fractionation. Accordingly, for example, the final photoconductor will contain less selenium on the top surface, which adversely affects the electrical characteristics thereof, and adds significantly to the cost of the process. With the processes of the present invention, these and other disadvantages are avoided.
Electrophotographic photoconductive imaging members containing amorphous selenium can be modified to improve panchromatic response, increase speed and to improve color copyability. These improved members are usually comprised of chalcogenide alloys such as alloys of selenium with tellurium and/or arsenic. The selenium imaging members may be fabricated as single layer devices comprising a selenium-tellurium, selenium-arsenic, selenium antimony, or selenium-tellurium-arsenic alloy layer which functions as a charge generation and charge transport medium. The selenium electrophotographic imaging members may also comprise multiple layers such as, for example, a selenium alloy transport layer and a continuous selenium alloy generator layer.
One known process for the preparation of photoconductors comprises the vacuum deposition of a selenium alloy on a supporting substrate such as aluminum. Tellurium can then be incorporated therein as an additive primarily for the purpose of enhancing the spectral sensitivity thereof. Also, arsenic can be incorporated as an additive for the primary purpose of improving wear characteristics, passivating against crystallization and improving electrical properties of the resulting photoconductor. Generally, the tellurium addition can be accomplished as a thin selenium-tellurium alloy layer deposited over a selenium arsenic alloy layer to achieve the benefits of the photogeneration and transport characteristics of these respective layers. Fractionation of chalcogenide alloys, such as tellurium and/or arsenic composition, during the vacuum evaporation processes results in an undesirable concentration gradient in the deposited photoconductor. Accordingly, there results inhomogeneities (fractionation) in the stoichiometry of the vacuum deposited thin film alloys. More specifically, fractionation occurs, it is believed, as a result of differences in the partial vapor pressure of the molecular species of the solid and liquid phases of binary, ternary and other multicomponent alloys. An important aspect in the generation of chalcogenide alloy imaging members is controlling the fractionation of alloy components, one of the advantages of the process of the present invention, such as tellurium and/or arsenic during the evaporation of source alloys. More specifically, tellurium and/or arsenic fractionation control is particularly important since the tellurium and/or arsenic concentration at the extreme top surface of the resulting photoreceptor effects xerographic sensitivity, charge acceptance, dark discharge, copy quality, photoreceptor wear, yield, crystallization resistance, and the like. For example, in single layer low arsenic selenium alloy photoreceptors arsenic enrichment at the top surface caused by fractionation can also cause severe reticulation of the evaporated film. Further, in single layer tellurium selenium alloy photoreceptors, tellurium enrichment at the top surface due to fractionation can cause undue sensitivity enhancement, poor charge acceptance and enhancement of dark discharge. Also, in two layer or multilayer photoreceptors where low arsenic alloys may be incorporated as a transport layer, arsenic enrichment at the interface with the layer above can lead to residual cycle up problems. Moreover, in two layer or multilayer photoreceptors where tellurium alloys may be incorporated as a generator layer, tellurium enrichment at the upper surface of the tellurium alloy layer can result in similar undue sensitivity enhancement, poor charge acceptance, and enhancement of dark discharge.
A specific method of preparing selenium alloys for evaporation comprises the grinding of amorphous selenium alloy shot (beads) and compressing the ground material into pellet agglomerates, typically from about 150 to about 300 milligrams in weight and having an average diameter of about 6 millimeters (6,000 micrometers). The pellets are then evaporated from crucibles in a vacuum coater with a time/temperature crucible program designed to minimize the fractionation of the alloy during evaporation. One disadvantage of the aforementioned vacuum deposited photoconductors, such as a selenium-tellurium alloy layer, is the crystallization of the selenium-tellurium alloy at the surface of the layer when exposed to heat. To retard premature crystallization and extend photoreceptor life, the addition of up to about 5 percent arsenic to the selenium-tellurium alloy can be beneficial without impairment of xerographic performance.
Also, in deposited layers of selenium-tellurium alloys the amounts of top surface tellurium present can cause excessively high photosensitivity. This photosensitivity is variable and changes as the surface of the layer wears away. Surface injection of corona deposited charge and thermally enhanced bulk dark decay involving carrier generation cause the toner images in the final copies to exhibit a washed out, low density appearance. Excessive dark decay causes loss of high density in solid areas of toner images and a general loss of image density. For example, when the photoreceptor comprises a single layer selenium arsenic alloy, about 1 to about 2.5 percent by weight arsenic based on the weight of the entire layer at the surface of the alloy layer, there is provided protection against surface crystallization. When the concentration of arsenic is greater than about 2.5 percent by weight, electrical instability risks increase.
One known method for attempting to control fractionation is the selection of shutters for incorporation over the evaporation crucibles. The tellurium or arsenic rich material originating from the crucible deposits on the shutter rather than on the photoreceptor substrate. However, in planetary coating systems, installation of shutters is complex, difficult and expensive. Further, after one or more coatings it may be necessary to clean the surface of the shutters and the resulting debris can cause defects to occur in subsequently formed photoreceptor layers.
Accordingly, a problem encountered in the fabrication of chalcogenide alloy photoconductors, such as selenium alloy photoreceptors, is the fractionation or preferential evaporation of an alloy component whereby the resulting film composition is not equivalent to the source alloy. Thus, the deposited film or layer does not have a uniform composition extending from one surface to the other. For example, when tellurium is the dopant, the tellurium concentration is high at the top surface and can approach zero at the bottom surface in contact with the substrate of the vacuum deposited layer. This problem is also observed for alloys of Se--Te, Se--As, Se--As--Te, Se--As--Te--Cl, or other halogens, mixtures thereof, and the like.
In copending application U.S. Ser. No. 946,238, filed 12-23-86, now U.S. Pat. No. 4,770,965, the disclosure of which is totally incorporated herein by reference, there is illustrated a process which includes heating an alloy comprising selenium and from about 0.05 percent to about 2 percent by weight arsenic until from about 2 percent to about 90 percent by weight of the selenium in the alloy is crystallized, vacuum depositing the alloy on a substrate to form a vitreous photoconductive insulating layer having a thickness of between about 100 micrometers and about 400 micrometers containing between about 0.3 percent and about 2 percent by weight arsenic at the surface of the photoconductive insulating layer facing away from the conductive substrate, and heating the photoconductive insulating layer until only the selenium in the layer adjacent the substrate crystallizes to form a continuous substantially uniform crystalline layer having a thickness up to about one micrometer. A thin protective overcoating layer can be applied on the photoconductive insulating layer. The selenium-arsenic alloy may be partially crystallized by placing the selenium alloy in shot form in a crucible in a vacuum coater and heating to between about 93.degree. C. (200.degree. F.) and about 177.degree. C. (350.degree. F.) for between about 20 minutes and about one hour to increase crystallinity and avoid reticulation. Preferably, the selenium-arsenic alloy material in shot form is heated until from about 2 percent to about 90 percent by weight of the selenium in the alloy is crystallized. The selenium-arsenic alloy material shot may be crystallized completely prior to vacuum deposition. However, if desired, a completely amorphous alloy may be used as the starting material for vacuum deposition. In Examples II and V of this copending patent application, halogen doped selenium-arsenic alloy shot containing about 0.35 percent by weight arsenic, about 11.5 parts per million by weight chlorine, and the remainder selenium, based on the total weight of the alloy, was heat aged at 121.degree. C. (250.degree. F.) for 1 hour in crucibles in a vacuum coater to crystallize the selenium in the alloy. After crystallization, the selenium alloy was evaporated from chrome coated stainless steel crucibles at an evaporation temperature of between about 204.degree. C. (400.degree. F.) and about 288.degree. C. (550.degree. F.).
In U.S. Pat. No. 4,780,386, the disclosure of which is totally incorporated herein by reference, illustrates a process wherein the surfaces of large particles of an alloy comprising selenium, tellurium and arsenic, the particles having an average particle size of at least 300 micrometers and an average weight of less than about 1,000 milligrams, are mechanically abraded while maintaining the substantial surface integrity of the large particles to form between about 3 percent by weight to about 20 percent by weight dust particles of the alloy based on the total weight of the alloy prior to mechanical abrasion. The alloy dust particles are substantially uniformly compacted around the outer periphery of the large particles of the alloy. The large particles of the alloy may be beads of the alloy having an average particle size of between about 300 micrometers and about 3,000 micrometers or pellets having an average weight between about 50 milligrams and about 1,000 milligrams, the pellets comprising compressed finely ground particles of the alloy having an average particle size of less than about 200 micrometers prior to compression. In one embodiment, the process disclosed in this patent comprises mechanically abrading the surfaces of beads of an alloy comprising selenium, tellurium and arsenic having an average particle size of between about 300 micrometers and about 3,000 micrometers while maintaining the substantial surface integrity of the beads to form a minor amount of dust particles of the alloy, grinding the beads and the dust particles to form finely ground particles of the alloy, and compressing the ground particles into pellets having an average weight between about 50 milligrams and about 1,000 milligrams. In another embodiment of the '386 patent, mechanical abrasion of the surface of the pellets after the pelletizing step may be substituted for mechanical abrasion of the beads. The process includes providing beads of an alloy comprising selenium, tellurium and arsenic having an average particle size of between about 300 micrometers and about 3,000 micrometers, grinding the beads to form finely ground particles of the alloy having an average particle size of less than about 200 micrometers, compressing the ground particles into pellets having an average weight between about 50 milligrams and about 1,000 milligrams, and mechanically abrading the surface of the pellets to form alloy dust particles while maintaining the substantial surface integrity of the pellets. Pellets of the present invention can be formulated as illustrated in the '386 patent.
In copending application U.S. Ser. No. 179,379 filed 4-8-88, now U.S. Pat. No. 4,822,712, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for controlling fractionation. More specifically, there is disclosed in this copending application processes for crystallizing particles of an alloy of selenium comprising providing particles of an alloy comprising amorphous selenium and alloying component selected from the group consisting of tellurium, arsenic, and mixtures thereof, said particles having an average size of at least about 300 micrometers and an average weight of less than about 1,000 milligrams, forming crystal nucleation sites on at least the surface of said particles while maintaining the substantial integrity of said particles, heating the particles to at least a first temperature between about 50.degree. C. and about 80.degree. C. for at least about 30 minutes to form a thin, substantially continuous layer of crystalline material at the surface of the particles while maintaining the core of selenium alloy in said particles in an amorphous state, and rapidly heating said particles to at least a second temperature below the softening temperature of said particles, the second temperature being at least 20.degree. C. higher than the first temperature and between about 85.degree. C. and about 130.degree. C. to crystallize at least about 5 percent by weight of said amorphous core of selenium alloy in the particles. With the process of the present invention, there is initially accomplished the crystallization or partial crystallization as illustrated, for example, in the appropriate aforementioned copending application.
Further, in copending application U.S. Ser. No. 261,659 filed 10-24-88, now U.S. Pat. No. 4,904,559 entitled Processes for Suppressing the Fractionation of Chalcogenide Alloys with the listed inventors Santokh S. Badesha, Paul Cherin and Harvey J. Hewitt, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for the preparation of chalcogenide alloy compositions which comprises providing a chalcogenide alloy; admixing therewith crystalline or amorphous selenium; and subsequently subjecting the resulting mixture to evaporation.
Also, in copending application U.S. Ser. No. 270,184 filed 11-04-88, now U.S. Pat. No. 4,894,307, the disclosure of which is totally incorporated herein by reference, there is described a process which comprises providing a chalcogenide alloy source material, crystallizing said source material, vacuum evaporating the source material, and adding in effective amounts thereto, prior to, during, or subsequent to evaporation, organic components such as siloxane polymers or greases, enabling the formation of a photoconductor with improved characteristics as illustrated herein. In one specific embodiment of the present invention, there is provided a process which comprises providing a chalcogenide alloy such as an alloy containing selenium, including selenium arsenic and selenium tellurium alloys; crystallizing said alloy; and vacuum evaporating the alloy by heating at a temperature of from about 250.degree. C. to about 350.degree. C. in the presence of an organic polymer such as a siloxane in an amount of from about 10 to about 30 parts per million, and depositing on a supporting substrate the desired chalcogenide alloy with reduced fractionation. Crystallization for the process of the present invention can be accomplished as illustrated in the aforementioned copending application.
Of particular interest with respect to the invention of the present application is copending application U.S. Ser. No. 07/179,193 filed Apr. 8, 1988 now U.S. Pat. No. 4,859,411, entitled Control of Selenium Alloy Fractionation, the disclosure of which is totally incorporated herein by reference, which describes and claims a process for crystallizing particles of an alloy of selenium comprising providing pellets of an alloy comprising amorphous selenium and an alloying component selected from the group consisting of tellurium, arsenic, and mixtures thereof, said pellets having an average weight between about 50 milligrams and about 1,000 milligrams, exposing said pellets to an ambient temperature of between about 114.degree. C. and about 190.degree. C. until an exotherm occurs in said pellets at between about 104.degree. C. and about 180.degree. C., carrying said exotherm through to substantial completion, grinding said pellets into fresh powder having an average particle size of less than about 200 micrometers, and compressing said fresh powder into fresh pellets having an average weight between about 50 milligrams and about 1,000 milligrams. With the present invention, in one important embodiment the crystallized source alloy is initially subjected to grinding and then pelletizing. A more complex process is, for example, disclosed in the '193 application since one has to initially prepare the pellets, and heat at higher deexotherm temperatures. On page 24 of the aforementioned application, it is indicated that surprisingly vacuum deposited layers formed directly from pellets that are heat treated by exposure to temperatures of between about 110.degree. and 190.degree. C. until an exotherm occurs in the pellet at between about 104.degree. and about 120.degree. C. are characterized by unaccepatably high photoreceptor sensitivity whereas vacuum deposited layers formed from pellets subjected to the same heat treatment followed by grinding and repelletizing are characterized by photoreceptor sensitivity within the specified values. With the process of the present invention low crystallization temperatures are preferably selected, for example, from about 85.degree. to about 100.degree. C., and preferably 95.degree. C. thereby enabling an alloy product with a different microstructure, presumably smaller crystallite size, which permits more desirable thermal properties, including thermal conductivity, and superior heat transfer from a alloy product particle to another alloy product particle providing for superior control of fractionation.
Prior art U.S. patents mentioned in the Ser. No. 07/179,193 copending application now U.S. Pat. Nos. 4,859,411, includes 4,205,098; 4,609,605; 4,297,424; 4,554,230; 3,524,745; 4,710,442; 4,583,608; 4,585,621; 4,632,849; 4,484,945; 4,414,179; 4,015,029; 3,785,806 and 3,911,091. Also, as prior art there is mentioned Swiss Patent Publication No. CH-656-486 published June 30, 1986; and Japanese Patent Publications Nos. J6 0172-346A and 57-91567.
There is illustrated in U.S. Pat. No. 4,513,031 a process for the formation of an alloy layer on the surface of a substrate, which for example comprises forming in a vessel a molten bath comprising at least one vaporizable alloy component having a higher vapor pressure than at least one other vaporizable alloy component in the bath; forming a thin substantially inert liquid layer of an evaporation retarding film on the upper surface of the molten bath, the liquid layer of the evaporation retarding film having a lower or comparable vapor pressure than both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component; covaporizing at least a portion of both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component whereby the evaporation retarding film retards the initial evaporation of the vaporizable alloying component having a higher vapor pressure; and forming an alloy layer comprising both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component on the substrate, see column 3, lines 33 to 54, for example. Examples of vaporizable alloying components include selenium-sulfur and the like; and examples of vaporizable alloying components having relatively low vapor pressures which include tellurium, arsenic, antimony, bismuth, and the like are illustrated in column 4, reference for example lines 41 to 50. Examples of suitable evaporation retarding film materials are outlined in column 4, at line 54, and continuing onto column 5, line 36, such materials including long chain hydrocarbon oils, inert oils, greases or waxes at room temperature which readily flow at less than the temperature of detectable deposition of the vaporizable alloying components having higher vapor pressures in the alloying mixture. Examples of retarding materials include lanolin, silicone oils such as dimethylpolysiloxane, branched or linear polyolefins such as polypropylene wax and polyalpha olefin oils, and the like, see column 5. According to the teachings of this patent, optimum results are achieved with high molecular weight long chain hydrocarbon oils and greases generally refined by molecular distillation to have a low vapor pressure at the alloy deposition temperature, see column 5, lines 32 to 36. It is believed with the aforementioned process that the levels of organics which are incorporated into the resulting alloy film may be sufficiently high causing negative adverse effects in the electrical properties of the resulting photoreceptor, for example, residual potential, dark decay, charge trapping, and cyclic stability are adversely effected. These and other problems are avoided with the process of the present invention. Moreover, there is no teaching in this patent with respect to initially crystallizing, or partially crystallizing the source component, and thereafter vacuum evaporating this component in the presence of a hydrocarbon, silicones, lanolins, which are a mixture of polyalcohols, polyesters, and fatty acids, especially the addition of these materials at low levels, that is from about 10 to about 30 parts per million thereby enabling photoreceptors with lower concentrations of organic materials thus permitting the advantages of the present invention, and avoiding those disadvantages as illustrated in the prior art such as the '031 patent, including, it is believed, the ineffective control of fractionation when low levels of organic additives, such as, for example, from about 10 to about 30 parts per million are selected.
More specifically, there is described in the aforementioned U.S. Pat. No. 4,205,098 a process wherein a powdery material of selenium alone or at least with one additive is compacted under pressure to produce tablets, the tablets being degassed by heating at an elevated temperature below the melting point of the metallic selenium and thereafter using the tablets as a source for vacuum deposition. The tablets formed by compacting the powdery selenium under pressure may be sintered at a temperature between about 100.degree. C. and about 220.degree. C. Typical examples of sintering conditions include 210.degree. C. for between about 20 minutes and about 1 hour, and about 1 to about 4 hours at 100.degree. C. depending upon compression pressure. Additives mentioned include Te, As, Sb, Bi, Fe, Tl, S, I, F, Cl, Br, B, Ge, PbSE, CuO, Cd, Pb, BiCl.sub.3, SbS.sub.3, Bi.sub.2 S.sub.3, Zn, CdS, CdSeS and the like.
Furthermore, in U.S. Pat. No. 4,609,605, the disclosure of which is totally incorporated herein by reference, there is illustrated a multilayered electrophotographic imaging member wherein one of the layers may comprise an amorphous selenium-tellurium-arsenic alloy prepared by grinding selenium-tellurium-arsenic alloy beads, with or without halogen doping, preparing pellets having an average diameter of about 6 millimeters from the ground material, and evaporating the pellets in crucibles in a vacuum coater; and in U.S. Pat. No. 4,297,424 there is described a process for preparing a photoreceptor wherein selenium-tellurium-arsenic alloy shot is ground, formed into pellets and vacuum evaporated. Further, in U.S. Pat. No. 4,780,386 there is illustrated a process for the preparation of electrophotographic imaging members which comprises providing large particles of an alloy of selenium, tellurium, arsenic, mechanically abrading the surfaces of the large particles whereby there is formed a certain amount of dust particles based on the total weight of the alloy prior to abrasion, reference the Abstract of the Disclosure.
There is illustrated in U.S. Pat. No. 4,554,230 a method for fabricating a photoreceptor wherein selenium-arsenic alloy beads are ground, formed into pellets and vacuum evaporated. Also, the following U.S. patents are mentioned: U.S. Pat. No. 3,524,754 directed to a process for preparing a photoreceptor wherein selenium-arsenic-antimony alloys are ground into fine particles and vacuum evaporated; U.S. Pat. No. 4,710,442 relating to an arsenic-selenium photoreceptor wherein the concentration of arsenic increases from the bottom surface to the top surface of the photoreceptor such that the arsenic concentration is about 5 weight percent at a depth of about 5 to 10 microns on the top surface of the photoreceptor and is about 30 to 40 weight percent at the top surface of the photoreceptor, which photoreceptor can be prepared by heating a mixture of selenium-arsenic alloys in a vacuum in a step-wise manner such that the alloys are consequentially deposited on the substrate to form a photoconductive film with an increasing concentration of arsenic from the substrate interface to the top surface of the photoreceptor. In one specific embodiment, a mixture of three selenium-arsenic alloys are maintained at an intermediate temperature in the range of from about 100.degree. to 130.degree. C. for a period of time sufficient to dry the mixture; and U.S. Pat. No. 4,583,608 relating to the heat treatment of single crystal superalloy particles by using a heat treatment cycle during the initial stages of which incipient melting occurs within the particles being treated. During a subsequent step in the heat treatment process, substantial diffusion occurs in the particle. In a related embodiment, single crystal particles which have previously undergone incipient melting during a heat treatment process are prepared by a heat treatment process.